Preheat oven to 180 C. Line an 8-by-8-inch baking tin with aluminium foil and grease.
In a microwave-safe bowl melt the butter, about 1 minute on high power.
Wait a minute before adding the egg so you don’t scramble it. Add the egg, brown sugar, vanilla, and whisk until smooth.
Add the flour and stir until just combined.
Stir in the milk chocolate chip chips.
Add the peanut butter and stir until evenly dispersed.
Turn batter out into prepared pan, smoothing the top lightly with a spatula.
To create the marbled pattern shown, drizzle the Nutella over the batter in three parallel lines, evenly spaced. Next rotate the pan 90 -degrees. With a toothpick, starting at the top of the pan, drag the toothpick through the Nutella 5/6 times.
Bake for about 20 to 25 minutes, or until done. A toothpick inserted in the centre should come out clean, or with a few moist crumbs.
Allow the blondes to cool in the baking tin for about 40minutes and then transfer to a wire rack to finish cooling. You can then cut into squares to serve.